Melting or softening of small particles under controlled conditions to convert them to generally ellipsoidal form is known. "Atomization," "fire polishing" and "direct fusion" techniques have been used.
Atomization methods involve first melting myriad raw material particles together to convert them to molten, i.e. bulk liquid, glass. Such bulk liquid typically contains far more than hundreds or thousands of times the amount of raw material required to make a single product particle. A thin stream of this molten glass is "atomized" by dropping it into a disruptive air jet, subdividing the stream into fine, molten droplets. The droplets are kept away from one another and from other objects until they have been cooled and solidified. Then they can be recovered as substantially discrete, generally ellipsoidal glass particles.
Describing atomization of glasses, Katz and Milewski, at page 303 of their "Handbook of Fillers and Reinforcements for Plastics," Van Nostrand Reinhold Company, New York, N.Y., 1978, explain that a glass batch, which initially includes crystalline materials, may contain sand, soda ash, dolomite, feldspar and other ingredients. When melted and thoroughly mixed so that the ingredients are no longer crystalline, the resultant bulk liquid material is then atomized. Glassy, amorphous, generally ellipsoidal particles are formed.
In fire-polishing, discrete solid particles of material having irregular shapes are heated to the softening or melting temperature of the material while suspended and dispersed in a hot gaseous medium. As particles become soft or molten, surface tension forms them into ellipsoidal shapes. If kept in suspension until cooled below the temperatures at which they "freeze" and become solid, the particles may then be recovered as generally discrete glassy ellipsoids.
Particulate feed materials for fire-polishing may be in the form of amorphous crushed glass solids when initially introduced into the gaseous medium. Thus, at page 302 of Katz and Milewski it is shown that particles of crushed and screened glass, such as plate glass, various glass cullets and bottle glass, all amorphous materials, may be suspended and dispersed in a hot gaseous medium and softened or melted to form then into ellipsoidal shapes.
Direct fusion bears some resemblance to fire-polishing. Feed particles with irregular shapes, including individual solid particles and/or adherent groups of such particles that are sometimes referred to as "clusters" or "agglomerates," are heated and softened or melted while in suspension and dispersion in a hot gaseous medium to form then into molten, generally ellipsoidal shapes, followed by cooling, freezing and recovery. Direct fusion draws its name in part from the fact that its feed particles directly undergo conversion to glassy or amorphous form in the ellipsoid-forming step, without prior conversion to bulk liquid form.
It is believed that a group of several mutually adherent feed particles, whether they become adherent prior to or during the ellipsoid-forming step, can melt and fuse to form a single, generally ellipsoidal particle of proportionately larger diameter. Thus, when these fused products are produced by direct fusion, whether they are formed from such groups of feed particles and/or from particles that remain discrete during fusion, the resulting generally ellipsoidal particles generally exhibit the varying average chemical compositions of the particles and/or groups of particles from which the ellipsoids are respectively formed, except that there may be relatively small losses of ingredients through high-temperature volatilization. Direct fusion products do not necessarily have the more uniformly similar particle-to-particle composition expected of particles produced from bulk liquid glass.
Atomization and fire polishing of glasses may be described as indirect methods. Their feed materials have been formulated from glass-making raw materials which were melted and homogenized in the form of bulk liquid prior to entering the ellipsoid-forming step. Consequently, in indirect methods, the individual chemical identities of the glass-making raw materials have been merged into an average composition which is uniformly present in the respective ellipsoids so produced.
Illustrations of direct fusion may be found in Japanese published patent applications HEI 21990! 59416 and HEI 21990! 199013, published respectively on Feb. 28, 1990 and Aug. 7, 1990. Therein, Morishita, et al and Shimada, et al respectively suggest fusing high purity silica particles with sizes measured in microns. The resultant products are for example useful as fillers in plastics.
Also, Klingaman and Ehrenreich, in U.S. Pat. Nos. 4,268,320 and 4,294,750, teach how to recover pyroplastoids, fused, substantially non-hollow alumino-silicate glassy ellipsoidal particles from fly ash found in the flue gases of coal fired boilers. These fused particles are also used as fillers in plastics, and for other purposes.
Ellipsoidal particles recovered from fly ash are generally economical, but can suffer from the disadvantage of containing colorants that are expensive if not virtually impossible to remove. Such colorants render these ellipsoids undesirable for certain end use applications.
Atomization processes can produce products comparatively free of undesirable colorants. However, these do not readily produce abundant quantities of some of the smaller particle sizes that are desired, for example particles smaller than 25 microns in average size.
Fire polishing of crushed or ground commercial glasses can be used to make very small particles having low color levels. However, the high cost of milling these amorphous materials to small sizes has contributed to the high cost of making small, uncolored particles by this route.
Direct fusion processes heretofore disclosed for converting crystalline silica to amorphous ellipsoids appear capable of producing white or transparent particles in very small sizes, but tend to be quite expensive due to the energy required to fuse these high-melting materials. It has been suggested that these processes be applied to broad categories of mineral materials, including alumino-silicates, metal silicates and other inorganic powders. However, whether this suggestion is practical, which of the myriad types and forms of raw materials available in these categories should be employed, and how this suggestion should be implemented to overcome the above difficulties, have yet to be made clear.
Thus, it is believed that a need remains for improvements in ellipsoidal fused particulate products, and in methods for producing them. The present invention seeks to fulfill this need.
Disclosure of the Invention in Summary Form PA0 Advantages
Fulfillment of this need has been accomplished in part by development of a method disclosed herein. It produces, in bulk, particulate material that includes solid, generally ellipsoidal particles. The method includes bringing into a dispersed condition irregularly shaped feed particles including about 60 to 100% by weight of at least one silicate-containing material selected from among wollastonite, alkali feldspar, plagioclase feldspar and nepheline. While maintaining the feed particles in dispersed condition, the feed particles are heated sufficiently to bring about at least partial fusion within at least the surfaces of the irregularly shaped particles. This produces a bulk particulate product in which about 15 to 100% by volume of the bulk particulate product is generally ellipsoidal particles.
The compositions of matter of the present invention comprise solid particles. In these compositions at least a portion of the particles are generally ellipsoidal particles that are substantially glassy. At least a portion of the particles respectively have chemical compositions corresponding substantially with that of material selected from among wollastonite, alkali feldspar, plagioclase feldspar and nepheline. The compositions of matter comprise about 15 to 100% by volume of said generally ellipsoidal particles that have said chemical compositions, based on the total volume of solid particles present in said compositions of matter.
The invention, depending on which of its various embodiments is used, is expected to provide one or more of the advantages set forth in this and succeeding paragraphs. It should be understood therfore that the invention includes embodiments which possess less than all of the advantages described below.
It is an advantage of the invention that the designated minerals, partly because of their crystalline structure, can be ground easily to an average size as small as 3 microns, and, when mined from appropriate deposits, can readily be freed to the extent necessary or desired from certain of the accessory minerals with which they are found combined in nature, such as magnetite.
Another advantage of the invention is that these minerals can be efficiently melted in an "open" flame, without special confining furnace walls or flame quenching processes, to provide generally ellipsoidal particles which are only a few microns in average particle size. Among the preferred feed materials of the invention are those which, as described below, can have relatively low viscosity at temperatures slightly above their crystalline dissolution temperatures, whereby surface tension can readily form the particles, when melted or softened, into generally ellipsoidal shapes. As compared to silica, the designated minerals can have a significantly smaller temperature difference between their ellipsoid-forming and "melting" temperatures. In fact, the preferred minerals can be converted to generally ellipsoidal form in high yields in an open flame of natural gas and air, without unwanted agglomeration.
It is surprising that the above described minerals could be successfully "flash fused" in the above manner, at flame temperatures comparable to those used in so-called "indirect" processes, in which the feed material is a glass powder, as distinguished from these crystalline feed materials. For example, wollastonite is reported to have a melting point of 1540.degree. C., which is at least about 400.degree. above the working temperature at which most commercial glasses are fire-polished to ellipsoidal form.
Also, it was not apparent that an unconfined or open flame would have sufficient heat capacity to successfully convert substantial proportions of feed particles of the designated minerals to generally ellipsoidal shape at the relatively low, energy conserving temperatures, for example about 1000 to about 1900.degree. C., that have been successfully used in the method of the present invention. Although it has been taught that dispersion of fine mineral particles in flames tends to extinguish them, due to lack of sufficient heat capacity in the flames, the method of the invention can be operated without undue difficulties.
While the inventor does not wish to be bound by any theory, it appears that the designated minerals may be fused with particular effectiveness when or as they contain substantial amounts of materials which cause them to deviate from their nominal chemical formulas, such as solid solutions, separate phases or small levels of ionic substitutions to be discussed in greater detail below. Those components of designated minerals that are responsible for such deviation may cause a lowering of feed particle melting points and working temperatures, as occurs in other crystalline materials which differ from their nominal composition.
It is also surprising that generally ellipsoidal products with specific gravities about 5 to about 15% lower than the specific gravities of the designated mineral feeds have been recovered. This provides an advantage of about 5 to about 15% both in manufacturing and in applications for the resultant powders.
A further advantage of the invention is the fact that the designated minerals can be converted to products of essentially the same particle size as the ground minerals used as feed materials for the fusion operation. More particularly, they can be readily converted to generally ellipsoidal particles under conditions that do not highly promote agglomeration of the product.
Another advantage of the invention is that the products can have higher temperature resistance than glass spheres manufactured by grinding and fusing various cullets, scrap window and bottle glasses and the like.
Products can be produced according to the invention for a wide variety of applications. For example, such products can be made in forms that are useful as film anti-blocking agents; as paint flatting agents; and as specialty powders useful in a wide variety of applications in thermosetting and thermoplastic resins such as silicones and fluoropolymers, in engineering plastics, in lotions and creams, and in composites, paper and other materials in any physical form, such as for instance molded products and single or multi-layer products including especially webs and laminates. They also are useful in powder form as anti-caking aids, and as a powder with unusual "slip" or lubricity.
The advantages of these products flow in part from the chemical composition of the feed materials and resultant products, and from the generally ellipsoidal shaped particles present in the products. These advantages are especially apparent in those products, now made economically available, which have very small particle diameters.
When produced from preferred ores, the products are characterized by high levels of whiteness and transparency, relatively low cost as compared to other generally ellipsoidal glassy fillers of comparable size, whiteness and transparency, and high chemical inertness. Moreover, the products can have essentially the same whiteness as the feed materials used. It is believed that the present invention represents the most cost effective means known for directly manufacturing small-diameter, substantially non-hollow, generally ellipsoidal particles with a high degree of whiteness and transparency.
When produced in forms characterized by sufficient amounts of generally ellipsoidal particles, e.g. about 30 or more and up to 100% by volume based on the total volume of the solids contents of the compositions, the products may be used, even at relatively high concentrations, to form relatively low viscosity mixtures in liquids or molten plastics. Products that are abundant in generally ellipsoidal particles can have high levels of hardness coupled with low abrasiveness. Highly ellipsoidal products are also characterized by relatively low surface area and consequently engage in relatively little surface interaction with other materials with which they may be formulated in a variety of end use applications.
Products containing some particles having significant surface roughness may for example be employed to advantage in compositions where some degree of abrasiveness is desired. Fusion operations conducted according to the invention can be readily controlled to produce predetermined proportions of both substantially glassy and rough, irregular crystalline particles in the particulate product, which can thus be used to impart a predetermined degree of abrasiveness in end use applications. Such products are especially conserving of energy since much higher production rates per unit of fuel consumption can be attained where only partial conversion to ellipsoidal particles is required.